A polycarbonate resin has excellent transparency, heat durability, mechanical strength, etc., and is therefore widely used in the fields of electric appliances, machines, automobiles, medical appliances, and the like. A polycarbonate resin is also widely used in various transparent members due to the above various excellent features. Above all, attempts are made to apply a polycarbonate resin to transparent members for vehicles aiming at a decrease in weight. Such transparent members for vehicles include a headlamp lens, a resin windowpane, a rear lamp lens, a meter cover, and the like. Characteristically, these members have complicated forms and large sizes, and molded articles therefor are required to have very high product quality.
When the above members are produced from a polycarbonate resin composition by an injection molding method, the following points may be problems in some cases.
That is, one problem is that it is difficult to obtain a molded article that has an excellent mold releasing property, has little internal strain and is free of the occurrence of cracking. More specifically, when a large amount of a mold release agent is added for imparting the resin with a good mold release property, a molded article is liable either to have an internal strain or to undergo cracking. When the amount of a mold release agent is decreased, the occurrence of cracking increases due to an increase in resistance against mold releasing. Further, the above transparent members are required to have transparency, durability against molding heat and weatherability.
The above internal strain of a molded article is a strain observed as a non-uniform shade portion through a polarizing plate. It is not reduced by any general annealing but, on the contrary, is made conspicuous in some cases (the above strain will be sometimes referred to as “strain-2” for convenience). The strain-2 is liable to occur when molding is carried out at a high temperature with a large molding machine having many residence portions.
A strain observable as a color change in a striped pattern or “dense and less dense” non-uniformity through a polarizing plate is a strain that can be reduced by annealing to some extent (this strain will be sometimes referred to as “strain-1” for convenience). The main factor of the above strain-1 is a strain which individual polymer molecule chains are caused to have due to thermal stress, and the like.
The above strain-1 causes the occurrence of cracking during hard-coating procedures or in long-term properties, so that it is required to reduce the strain-1. While the strain-2 does not cause such problems as visibility when a windowpane or the like is used under natural light, it is sometimes required to reduce the strain-2. The reason therefor will be discussed later.
Further, a transparent member for a vehicle is often subjected to surface treatments such as hard coating treatment. In the hard coating treatment, a molded article sometimes undergoes cracking. The cracking of a molded article will take place even if the molded article is annealed before the hard coating treatment. It is therefore considered that some factors of a polycarbonate resin composition promote the cracking of a molded article during molding and annealing. It is hence required to improve the molded article in resistance to cracking (to be sometimes referred to as “cracking resistance” hereinafter) by reducing the above factors.
As described above, for members for vehicles, there is demanded a polycarbonate resin composition which has excellent transparency and durability against molding heat and gives a molded article having an excellent mold release property, a reduced internal strain and improved cracking resistance and preferably further having weatherability.
The manufacturing of transparent members for vehicles, particularly large members, has features different from the manufacturing of optical disk substrates that are typical molded articles from a polycarbonate resin. The manufacturing of an optical disk substrate is “molding” requiring a very high processing temperature but “molding” of an article having a simple form (a constant thickness and non-complicated flowing of a resin) at a very small cycle. In view of the above points, a resin composition therefor is required to have properties different from those of a resin composition for an optical disk substrate.
Conventionally, a method of adding a fatty acid ester is known as a method of improving the mold release property of a polycarbonate resin, and above all, glycerin monostearate is frequently used. In a polycarbonate resin composition containing glycerin monostearate, however, the above strain-2 is clearly observed, and such a composition cannot be said to be satisfactory in cracking resistance.
As a mold release agent for use in a polycarbonate resin, there is widely known a full ester of an aliphatic polyhydric alcohol and an aliphatic carboxylic acid, such as pentaerythritol tetrastearate. For a polycarbonate resin composition containing the above full ester, there are various proposals made for improving the composition in product quality. The full ester of an aliphatic polyhydric alcohol and an aliphatic carboxylic acid will be sometimes referred to as “fatty acid full ester” hereinafter.
There is disclosed a polycarbonate resin composition containing, as a mold release agent, an ester of pentaerythritol in which the OH group content and acid value of an ester compound are extremely decreased (Patent Document 1).
Further, there is disclosed a polycarbonate resin composition comprising a polycarbonate resin and an internal mold release agent having a full ester content of 90% or more and having an acid value of 0.6 to 1.6, an iodine value of 0.1 to 1.3 and a metal element, Sn, content of 5 to 300 ppm (Patent Document 2).
(Patent Document 1)
JP-A-2-69556
(Patent Document 2)
JP-A-2001-192543
(Objects to be Achieved by the Invention)
It is an object of the present invention to provide a polycarbonate resin composition which has excellent transparency and durability against molding heat, which gives a molded article having an excellent mold release property, reduced internal strain and improved cracking resistance and, preferably, further having weatherability, and which is suitable particularly for a transparent member for a vehicle, pellets thereof and a molded article thereof.
The present inventors have made diligent studies for achieving the above objects.
First, the above problem is that the mold release property imparted by a mold release agent and other properties are not compatible, so that the present inventors have studied types of mold release agents as one solution factor.
Second, the present inventors have made studies causes for cracking. As a result, it has been assumed that a cracking during molding is possibly a solvent cracking caused by the contact of additives partly remaining on a mold surface, such as a mold release agent, etc., to a high-temperature molded article during molding. Particularly, it has been found that since an edge portion of a molded article is liable to have a greater strain, and since a mold release agent, etc., are liable to be deposited in the edge portion, a cracking is liable to occur. While it is considered that a cracking also takes place in hard coating treatment after annealing mainly because a residual stress is not completely removed by the annealing, it is also considered from the above viewpoint that a resin of a molded article surface is partly deteriorated. Therefore, the conclusion obtained concerning these points is that the type of a mold release agent is also essential.
Third, studies have been made with regard to causes for the occurrence of the strain-2. As a result, the following conclusion has been reached. In injection molding, a resin flows into a mold in a fountain flow manner, and the mold is filled with the resin in a manner in which a resin to follow pushes aside a resin that has already flowed in the mold. For example, when the cross-section of the resin is observed, there is observed a state where several flow layers of the resin are stacked. In an injection-molded article having the above properties, when a non-uniform frictional force is caused among the flow layers, the flows of the resin are disordered, there is caused a great difference in density among the flow layers, or the flows of the resin is whirled to reverse upper and lower layers in some cases. It has been found that such states are observed as a non-uniform shade portion in the above observation through a polarizing plate. That is, the non-uniform shade can be said to be a strain of flow layers of a resin. It has been also found that the strain-2 is liable to take place when a thermal load on a resin is excessive. It has been therefore assumed that the non-uniform frictional force among the flow layers is caused by gasification of a decomposition product. Further, it has been found that as the strain-2 becomes more intensely observable, there are more cases where a molded article show defects such as sliver streaks, discoloration and internal fogging (white haze) due to a slight increase in residence time. That is, it can be said that a resin composition having less strain-2 causes almost no molding defects and can be applied widely to a variety of molded articles. Further, when long-term properties are taken into account, molded articles having less strain are more preferred than those having intense strains. When a molded article is used in place of a glass product, when it is applied to an optical product, and when it is applied to an automobile part, the strain-2 is often regarded as a defect. For the above reason, it is sometimes required to decrease the strain-2.
Therefore, to decrease the strain-2, the conclusion that has been reached is that the flow of a resin is required to be smooth and that it is required to incorporate an additive from which a gas and a decomposition substance are less generated. A mold release agent is considered to have the function of making the flow of a resin smooth, and it is considered to be essential to incorporate a mold release agent from which a gas and a decomposition product are less generated.
Following the above studies, the present inventors have further made diligent studies, and as a result, have found that a resin composition prepared by incorporating a specific amount of a specific fatty acid full ester to a polycarbonate resin can overcome the above problems, and the present invention has been accordingly completed.